Environmental Engineering Reference
In-Depth Information
likely to continue to be a major contributor to anthropogenic greenhouse gases [3,
4]. Mitigation of CO 2 -based contributions to the global warming process requires
specific actions, including capture and sequestration of CO 2 during the consump-
tion of fossil fuels and expanded utilization of carbon-neutral and carbon negative
renewable energy sources (wind, solar, nuclear, geothermal, and various biomass
sources) [3-6]. Most of the types of renewable energy (wind, solar, etc.) can be uti-
lized to generate electricity, but not liquid transport fuels. Consequently, biomass
has received much attention as a feedstock for biofuels, both in the existing com-
mercial industry (e.g. ethanol from grains or sugar) and in the research realm where
lignocellulose is the current focal feedstock material [7-11]. To avoid confusion, we
adapt the common definition for biomass and biofuels as follows:
Biomass : Organic, non-fossil material of biological origin (plant parts including
grains, tubers, stems/leaves, roots/tubers, agricultural residues, forest residues,
animal residues, and municipal wastes arising from biological sources) poten-
tially constituting a renewable energy source (basically originating from primary
capture of solar energy).
Lignocellulosic biomass : Organic material derived from biological origin which
has a relatively high content of lignin, hemicellulose, cellulose, and pectin com-
bined into a molecular matrix with a relatively low content of monosaccharides,
starch, protein, or oils. Typically refers to plant structural material with high cell
wall content. Sometimes referred to as “cellulosic” biomass, which is techni-
cally inaccurate, but is (mis)used due to the typical 40%+ cellulose content in
Biofuels : Liquid fuels and blending components produced from biomass (plant)
feedstocks, used primarily for transportation. Technically, biogas (e.g. methane
from anaerobic digestion of biological residues) is a “biofuel” but tends to be
utilized in stationary combustion units and is typically referred to separately as
Survey reports suggest that the annual world biomass yield contains sufficient
inherent energy to contribute 20-100% of the world's total annual energy con-
sumption of 500 EJ (1 EJ
10 18 Joule), with annual and regional variations
[4, 10, 12]. Currently, commercial biofuels are generated from harvestable compo-
nents of known crops (starch, sucrose, and oils), while a relatively small amount
of the lignocellulosic biomass is used for combustion (cooking/heating fires or co-
firing to create steam for electricity generation). The large potential of lignocellulose
as an energy feedstock remains to be utilized, and is dependent on the development
of economic, sustainable production, and processing systems [11].
Two platforms have been set up to transform the energy in lignocellulosic
biomass into liquid fuels or chemicals: the sugar platform and thermochemical
platform. In the sugar platform, the lignocellulosic material is first pre-treated to
facilitate separation into the major components, then the polymeric celluloses and
hemicelluloses are enzymatically hydrolyzed into sugars (hexoses and pentoses),
after which these sugars can be fermented into biofuels or converted into other
valuable intermediate chemicals. The residual lignin may be utilized as a specialty
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